Copper indium gallium selenide (cigs) thin films with composition controlled by co-sputtering

ABSTRACT

A method and apparatus for forming a thin film of a copper indium gallium selenide (CIGS)-type material are disclosed. The method includes providing first and second targets in a common sputtering chamber. The first target includes a source of CIGS material, such as an approximately stoichiometric polycrystalline CIGS material, and the second target includes a chalcogen, such as selenium, sulfur, tellurium, or a combination of these elements. The second target provides an excess of chalcogen in the chamber. This can compensate, at least in part, for the loss of chalcogen from the CIGS-source in the first target, resulting in a thin film with a controlled stoichiometry which provides effective light absorption when used in a solar cell.

This application is a divisional application of U.S. application Ser.No. 12/884,524, entitled Copper Indium Gallium Selenide (CIGS) ThinFilms with Composition Controlled by Co-Sputtering, filed Sep. 17, 2010by Jesse A. Frantz, which claimed the benefit of U.S. ProvisionalApplication Ser. No. 61/245,400, filed Sep. 24, 2009, entitled Thin FilmCu(In_(1-x)Ga_(x))Se₂ (0≦x≦1) with Composition Controlled byCo-Sputtering, by Jesse A Frantz, et al., and U.S. ProvisionalApplication Ser. No. 61/245,402, filed Sep. 24, 2009, entitled LowTemperature and High Temperature Synthesis of High-Purity BulkCu(In_(1-x)Ga_(x))Se₂ (0≦x≦1) Materials, by Vinh Q. Nguyen, et al., thedisclosures of which are incorporated herein in their entireties, byreference.

CROSS REFERENCE

Cross reference is made to copending application Ser. No. 12/884,586,filed Sep. 17, 2010, entitled SYNTHESIS OF HIGH-PURITY BULK COPPERINDIUM GALLIUM SELENIDE MATERIALS, by Vinh Q. Nguyen, et al., (AttorneyDocket No. 99918-US2) the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND

The present exemplary embodiment relates to a method for forming acompound semiconductor thin-film. It finds particular application inconjunction with semiconductor thin-films suitable for use inphotovoltaic solar cells and other devices, and will be described withparticular reference thereto. However, it is to be appreciated that thepresent exemplary embodiment is also amenable to other likeapplications.

Photovoltaic devices represent one of the major sources ofenvironmentally clean and renewable energy. They are frequently used toconvert solar energy into electrical energy. Typically, a photovoltaicdevice is made of a semiconducting junction with p-type and n-typeregions. The conversion efficiency of solar power into electricity ofsuch devices is limited to a maximum of about 30%, since photon energyin excess of the semiconductor's bandgap is wasted as heat and photonswith energies smaller than the bandgap do not generate electron-holepairs. The commercialization of photovoltaic devices depends ontechnological advances that lead to higher efficiencies, lower cost, andstability of such devices.

The cost of electricity can be significantly reduced by usingphotovoltaic devices constructed from inexpensive thin-filmsemiconductors. Thin films of polycrystalline copper indium galliumselenide of the form Cu(In_(1-x)Ga_(x))Se₂, 0≦x≦1 (CIGS), have shownpromise for applications in thin film photovoltaics. The band gaps ofthese materials range from approximately 1.1 to 1.7 eV (see, J. L. Shayand J. H. Wernick, “Ternary Chalcopyrite Semiconductors: Growth,Electronic Properties and Applications,” Pergamon, New York (1975)).This should allow efficient absorption of solar radiation. A solar cellwith an efficiency of 19.9%, measured with AM1.5 illumination, hasrecently been demonstrated by Repins, et al. (I. Repins, et al.,“19.9%-efficient ZnO/CdS/CuInGaSe₂ solar cell with 81.2% fill factor,”Progress in Photovoltaics: Research and Appl., 16, 235-239 (2008)). Seealso, K. W. Mitchell, Proc. 9^(th) E. C. Photovoltaic Solar EnergyConference, Freiburg, FRG, September 1989, p. 292. Kluwer, Dordecht(1989); M. A. Green, et al., Prog. Photovolt. Res. Appl. 15, 35 (2007);Report on the Basic Energy Sciences Workshop on Solar EnergyUtilization, US Dept. of Energy, Apr. 18-21, 2005; J. D. Beach, B. E.McCandless, Mater. Res. Bull. 32, 225 (2007); and M. A. Contreras, etal., Pro. Photovolt. Res. Appl. 13, 209-216 (2005).

CIGS films have been vacuum deposited by several different methods.These include evaporation (see, Repins, et al.), two-stage processesutilizing evaporated or sputter deposited precursors followed byselenization in H₂Se (see B. M. Basol, “Preparation techniques for thinfilm solar cell materials: processing perspectives,” Jph. J. Appl. Phys.32, 35 (1993); E. Niemi and L. Stolt, “Characterization of CuInSe₂ thinfilms by XPS,” Surface and Interface Analysis 15, 422-426 (1990)),metallic ink coating (G. Norsworthy, et al., “CIS film growth bymetallic ink coating and selenization,” Solar Energy Materials & SolarCells 60, 127-134 (2000)), and coating via soluble hydrazine-basedprecursors (D. B. Mitzi, et al., “A high-efficiency solution-depositedthin-film photovoltaic device,” Adv. Mater. 20, 3657-3662 (2008)).

While such techniques have produced efficient devices in the laboratory,there remains a need for CIGS deposition technologies that are scalableto large-area devices for commercial applications. Techniques forsputter deposition of CIGS, for example, have included the costly andpotentially hazardous step of further selenization in H₂Se of thepreviously sputtered elements. Films made by sputtering directly fromthe CIGS compounds are Se-poor since selenium is lost in the vapor phaseduring film deposition (see, V. Probst, et al., “Rapid CIS-process forhigh efficiency PV-modules: development towards large area processing,”Thin Solid Films, 387, 262-267 (2001)). Additionally, the morphology isvery coarse. Conventional sputtered CIGS films are thus typicallyunsuitable for high efficiency photovoltaic devices.

REFERENCES

The following references, the disclosures of which are incorporatedherein by reference in their entireties, are mentioned:

U.S. Pub. No. 2010/0159633, published Jun. 24, 2010, entitled METHOD OFMANUFACTURING PHOTOVOLTAIC DEVICE, by Byoung-Kyu Lee, et al., disclosesa method of manufacturing a photovoltaic device using a Jouleheating-induced crystallization method. The method includes: forming afirst conductive pattern on a substrate; forming a photoelectricconversion layer on the substrate having the first conductive pattern;and crystallizing at least part of the photoelectric conversion layer byapplying an electric field to the photoelectric conversion layer,wherein the photoelectric conversion layer includes a first amorphoussemiconductor layer containing first impurities, a second intrinsic,amorphous semiconductor layer, and a third amorphous semiconductor layercontaining second impurities.

U.S. Pub. No. 2009/0250722, published Oct. 8, 2009, entitled METHOD FORFORMING A COMPOUND SEMI-CONDUCTOR THIN-FILM, by Allan James Bruce, etal. discloses a method for fabricating a thin film semiconductor device.The method includes providing a plurality of raw semiconductormaterials. The raw semiconductor materials undergo a pre-reactingprocess to form a homogeneous compound semiconductor target material.The compound semiconductor target material is deposited onto a substrateto form a thin film having a composition substantially the same as acomposition of the compound semiconductor target material.

BRIEF DESCRIPTION

In accordance with one aspect of the exemplary embodiment, a method forforming a film includes providing first and second targets in a commonsputtering chamber, the first target including a source of CIGS materialand the second target including a chalcogen. The method further includessputtering the targets towards a substrate to provide a film comprisingpolycrystalline CIGS material on the substrate which incorporateschalcogen from the second target.

In accordance with another aspect of the exemplary embodiment, asputtering apparatus includes a sputtering chamber. At least a firsttarget is positioned in the chamber. The first target is energizable tosputter a source of a CIGS material towards a substrate. A second targetis positioned in the chamber, which is energizable to sputter achalcogen towards the substrate. By energizing the first and secondtargets, a layer of CIGS material is provided on the substrate, thelayer incorporating, within the CIGS material polycrystalline structure,chalcogen from the second target.

In accordance with another aspect of the exemplary embodiment, a methodfor forming a solar cell includes sputtering first and second targetstowards a substrate to provide a film comprising polycrystalline CIGSmaterial on the substrate, the first target including a source of CIGSmaterial having an oxygen concentration of less than 10 ppm and thesecond target including a chalcogen, the chalcogen in the second targetbeing at a higher concentration than in the first target, and the secondtarget also having oxygen concentration of less than 10 ppm by weight.First and second electrode layers are provided on the substrate. Theelectrode layers are spaced by the film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an co-sputtering apparatus in aconfiguration in accordance with one aspect of the exemplary embodiment;

FIG. 2 is a schematic diagram of an co-sputtering apparatus in aconfiguration in accordance with another aspect of the exemplaryembodiment;

FIG. 3 is a schematic cross sectional diagram of a thin filmsemiconductor device in accordance with another aspect of the exemplaryembodiment;

FIG. 4 is an SEM image of a thin film obtained by sputtering CIGSwithout co-sputtering of selenium;

FIG. 5 is an SEM image of a thin film obtained by sputtering CIGS withco-sputtering of selenium;

FIG. 6 is an XPS plot illustrating concentrations with depth through theexemplary film obtained by maintaining a constant power through aselenium target and a CIGS target; and

FIG. 7 is an XPS plot illustrating concentrations with depth through theexemplary film obtained by maintaining a constant power through a CIGStarget while varying power to a selenium target.

DETAILED DESCRIPTION

Aspects of the exemplary embodiment relate to a method and an apparatusfor preparing thin films of polycrystallinecopper-indium-gallium-selenium (CIGS) material and related compoundsconsisting essentially of copper, group IIIA metal, chalcogen, andoptionally one or more dopants, all of which will be referred to hereinfor convenience as CIGS materials. The exemplary method includesdepositing a CIGS material, while providing an excess of selenium, orother chalcogen in a sputtering chamber, to compensate for loss ofchalogen as vapor. The film of CIGS material produced by the method canthus comprise a CIGS material sputtered from a first target whichincorporates, within its polycrystalline structure, chalcogen which hasbeen sputtered from a second target.

The exemplary thin film can be wholly or partially made of apolycrystalline CIGS material which has the general formulaCu(In_(1-x)Ga_(x))M_(y), where 0≦x≦1, M represents a chalcogen selectedfrom selenium (Se), sulfur (S), tellurium (Te), and combinationsthereof, and y is approximately 2. The indium/gallium can be partiallysubstituted with Al. In one embodiment, the chalcogen in the CIGSmaterial is at least predominantly Se (i.e., >50 atomic % Se), and canbe at least 95 atomic % Se and up to 100 atomic % Se. While thechalcogen may be referred to herein as selenium, it is to be appreciatedthat the other chalcogens, S and Te, may alternatively or additionallybe present in the thin film.

While the CIGS material in the film can be stoichiometric (y=2), it isalso contemplated that the proportions of the elements may be variedfrom the exact stoichiometric amounts without loss of the integrity andhomogeneous dispersion of the elements in the crystalline material. Forexample, Se can vary from 80-120% of its stoichiometric amount in theCIGS material. An approximately stoichiometric formula of the CIGSmaterial can thus be represented by the general formula: Cu_(a)(Z)M_(y),where Z is selected from In, Ga, Al, and combinations thereof, 0.8≦a≦1,and 1.6≦y≦+2.4. In one embodiment, a=1 and y is approximately 2. TheCIGS material in the film can contain dopants, such as one or more ofNa, Li, Cr, and Ti. The dopant or dopants may each be at a concentrationof from 0.001 to about 2 atomic % of the film. The total of all dopantsin the film is less than 10 atomic %. The film may have a low oxygenconcentration, e.g., 10 ppm or less, as measured by inert gas fusionwith a LECO analyzer, available from LECO Corp., St. Joseph, Mich. Inthe exemplary embodiment, elements other than Cu, Al, In, Ga, andchalcogen account for no more than 10 atomic % of the film. Theapproximately stoichiometric CIGS materials in the film still have theCIGS crystalline structure, rather than different crystalline phases,but can have, for example, up to 1 in 5 vacancies, when copper isdeficient, and/or occasional sites occupied by dopant atoms.

The composition of the polycrystalline film is controlled byco-sputtering from separate targets, a first target (CIGS target)comprising a CIGS source material, and a second target (chalcogentarget), which is chalcogen rich, as compared to the first target, andcan be a pure polycrystalline chalcogen. The CIGS source material may bea CIGS material, as described above and/or one or more precursors forforming a CIGS material. The chalcogen in the chalcogen target isselected from selenium, sulfur, tellurium, and combinations thereof. Thecombination of targets can provide a higher dose of chalcogen in thesputtering chamber than is obtained with a single CIGS target. As aconsequence, the selenium or other chalcogen which would otherwise belost from the CIGS source material in the vapor phase is compensated forby the chalcogen in the second target. The resulting thin film can thusbe higher in chalcogen than it would be otherwise. By tailoring thecompositions of targets and/or their relative sputtering rates toprovide a higher than stoichiometric amount of chalcogen in the combinedsputtered material, the deposited film may have a more nearlystoichiometric polycrystalline CIGS structure, with few crystal defectsdue to missing chalcogen atoms.

A first embodiment of a co-sputtering apparatus 1 is illustrated inFIG. 1. The apparatus includes a first target 10 comprising a source ofthe CIGS material and a separate second target 12 comprising a chalcogenselected from selenium, sulfur, tellurium, and combinations thereof. TheCIGS source material in target 10 can be a polycrystalline CIGS materialas described above. The target 10 is mounted in a first sputter source14 and the target 12 is mounted in a second sputter source 18.

The sputtering apparatus 1 may be configured for any suitable sputteringmethod, such as radiofrequency (RF), DC, or magnetron sputtering. Forexample, the targets 10, 12 may be powered by separate RF, AC, or DCpower sources 22, 24, respectively. The power sources supply current toraise each target to a negative potential (cathode), causing material tobe sputtered off into a surrounding plasma. An RF magnetron (not shown)or positive electrode may be positioned adjacent the target. The targets10, 12 are arranged in a sputtering chamber 26 so that plumes 28, 30 ofsputtered material are coincident in a region 32 of the chamber adjacenta surface 34 of a substrate 36. For example, the sputter sources areangled towards the same area 38 on the substrate surface 34, so that theplumes overlap in area 38. This results in the deposition of a thinlayer film 40 (not to scale) of CIGS material. The chamber 26 isprovided with an inlet 42 for introducing an inert sputtering gas, suchas argon, at low pressure. The chamber is evacuated with a vacuum source(not shown) via an outlet 44. Walls of the chamber may be grounded.

One or both of the exemplary power sources 22, 24 is/are variablyadjustable by respective controllers 46, 48 for variably adjusting thepower to each of the target electrodes 10, 12. A ratio of energy (W/cm²)supplied to the surfaces of first and second targets 10, 12 can bevaried during the sputtering by variably controlling one or both powersources and/or by sputtering the two targets sequentially. In this way,the relative amounts of material sputtered from the targets may beadjusted during deposition of the film 40 to vary the concentration ofthe target materials in the film.

An electrically powered heater 50 variably adjusts the temperature ofthe substrate 36 to provide a suitable substrate surface temperature fordeposition of CIGS material (e.g., at least about 250°, such as about550° C.) and optionally to provide a suitable temperature for annealingthe film 40 for a period of time (e.g., a temperature of about 400-600°C., or higher if the substrate melting temperature is higher). Thesubstrate may be mounted for rotation on a substrate support 52. Thesupport may be coupled to an RF power source, a DC power source, an ACpower source, or to ground.

While FIG. 1 illustrates a sputter up configuration (targets positionedbelow the substrate), other configurations, such as sputter down, arealso contemplated.

The exemplary method for forming the CIGS film 40 includes mountingtargets 10, 12 and a substrate 36 in a common chamber 26. The chamber isflushed with argon, or other inert sputtering gas, and evacuated to alow argon pressure. Thereafter, while maintaining the chamber undervacuum with a slight argon gas pressure to provide argon gas flowingthrough the chamber, the method includes sputtering CIGS material fromtarget 10 and sputtering chalcogen from target 12. The energy applied tothe targets may be selected to provide an excess of chalcogen in thevapor. The energy applied to the targets can be different. For example,the energy applied in W/cm² is higher for target 10 than for target 12,e.g., at least two times that for target 12. The chalcogen in the vapormay be at least about 5 or 10 atomic % in excess of the CIGSstoichiometric amount, e.g., up to about 15%. By maintaining thesubstrate at a temperature above the melting point of selenium (219°C.), or other chalcogen, this excess does not contribute to depositionof a selenium phase. Rather, the film formed is polycrystalline CIGS.

The targets 10, 12 may be sputtered sequentially and/orcontemporaneously during the formation of the film 40. In the case ofsequential sputtering, the CIGS material in the film 40 may be formed byannealing, e.g., at about 400-600° C., sequentially-applied CIGS andchalcogen layers on the substrate at a sufficient temperature for thechalcogen to diffuse from the chalcogen layer(s) through the film to theCIGS layer(s) and react with the CIGS material to increase thestoichiometric amount of chalcogen in the polycrystalline CIGS material.One or more additional layers may be deposited over the thin film in theformation of a semiconductor device. The additional layer(s) may beformed in the same sputtering chamber or separately. The thin film 40may be of any suitable thickness, which may depend on the applicationfor which it is being used. For example, the thin film may be at least 5nanometers in thickness and can be up to about 500 nm or more.

As will be appreciated, a purely stoichiometric CIGS film 40 is 50atomic chalcogen (e.g., 2 Se atoms to one each of Cu and Ga/In) and hasa Ch:(Ch+Cu+Ga/In/Al) atomic ratio of 1:2, although this ratio may bemodified slightly in the exemplary film by the presence of one or moredopants which occupy sites in the tetragonal crystalline structure or bypermitting vacancies to occur.

In one exemplary embodiment target 10 is predominantly CIGS material(Ch+Cu+Ga/In/Al), e.g., at least 80 atomic % or at least 90 atomic % andcan be up to 100 atomic % (Ch+Cu+Ga/In/Al). Expressed as atomic percent,the target 10 can be at least 20 atomic % Cu, at least 20 atomic %Ga/In/Al, and at least 40 atomic % Ch (Ch=chalcogen, i.e., Se, S, and/orTe). The Ch:(Ch+Cu+Ga/In/Al) atomic ratio in target 10 may beapproximately 1:2, e.g., from 1:1.8 to 1:2.2, and in one embodiment,from 1:1.9 to 1:2.1. In the exemplary embodiment, the CIGS target 10 isup to about 50 atomic % chalcogen.

The second target 12 is chalcogen rich, relative to target 10, i.e., hasa Ch:(Ch+Cu+Ga/In/Al) atomic ratio which is higher than in the firsttarget. As a result of the excess chalcogen sputtered from target 12,the CIGS material in the film 40 has fewer crystal defects arising fromabsent chalcogen atoms. The Ch:(Ch+Cu+Ga/In/Al) atomic ratio in target12 may be at least 1:1.6, e.g., at least 1:1.5, and can be up to 1:1. Inthe exemplary embodiment, the chalcogen target 12 is predominantlychalcogen, i.e., greater than 50 atomic % chalcogen, e.g., at least 60atomic % chalcogen, and can be substantially pure chalcogen, e.g., atleast 80 or 90 atomic % chalcogen, or pure chalcogen, e.g., at least 99atomic % chalcogen, and can be up to 100 atomic chalcogen. A ratio ofchalcogen in the second target 12 to chalcogen in the first target 10may be at least 1.1:1, e.g., at least 1.2:1 and in some embodiments isat least 1.5:1 or at least 1.8:1 and can be up to about 2:1. Bothtargets 10, 12 may have an oxygen concentration of less than 10 ppm, byweight.

In some embodiments, the target 12 may include other materials, such asdopants, e.g., Na, Li, Cr, Ni, Ti, or a combination thereof. The dopantsmay be present, for example, at a total of up to 40 or 50 atomic % ofthe chalcogen target 12 to provide dopant in the film 40 at up to 10atomic % in total. In other embodiments, the dopant(s) may beco-sputtered from target 10 and/or a separate target.

Sputtering from two targets 10, 12 with different chalcogenconcentrations allows the ratios of the different constituents of thelayer 40 to be varied. This can be controlled by adjusting theelectrical power applied to the respective targets. For example, thesecan be tuned so that the resulting film 10 is closer to thestoichiometric amounts of the CIGS constituents. Additionally, oralternatively, the composition of the film 40 can be varied with depththrough the film's thickness by varying the power/controlling therelative power applied to each target during deposition. This method canbe applied, for example, to create a film with a tailored bandgap. Forexample, one region of the film may have a first Ch:(Ch+Cu+Ga/In/Al)atomic ratio and a second region at a different depth may have a secondCh:(Ch+Cu+Ga/In/Al) atomic ratio which is at least 5% or at least 10%greater or less than the first ratio.

While the co-sputtering can be performed with a single CIGS target 10and a single chalcogen target 12, in another embodiment, two (or more)CIGS targets 10A, 10B are provided, as illustrated in FIG. 2. Each CIGStarget 10A, 10B has a different CIGS stoichiometry. The two targets maybe sputtered using the same or separate power sources and controllers.

In one embodiment, a first CIGS target 10A may have a first value of xand a second CIGS target 10B may have a second value of x different fromthe first target. As an example, first CIGS target 10A may be formedfrom CuInSe₂ (x=0) and second CIGS target 10B is formed from CuGaSe₂(x=1).The In/Ga ratio in the deposited film may be controlled byadjusting the relative power to the respective sputtering guns.

In another embodiment, one target 10A may be formed from a CIGS compoundcomprising a first chalcogen, while the second target 10B uses a secondchalcogen. As an example, first CIGS target 10A may be formed fromCuInSe₂ and second CIGS target 10B is formed from CuInTe₂.

In another embodiment, one target 10A may be formed from a CIGS compoundcomprising a first value of x and first chalcogen, while the secondtarget 10B uses a second value of x and a second chalcogen. As anexample, first CIGS target 10A may be formed from CuInSe₂ and secondCIGS target 10B is formed from CuGaTe₂.

In another embodiment, the targets 10A and 10B each comprise arespective CIGS precursor, which when combined, form the exemplary CIGSmaterial. Co-sputtering from compounds that each form a subset of CIGScompounds may be used (e.g., CuSe₂, In₂Se₃, Ga₂Se₃). As an example,first CIGS target 10A may be formed from CuSe₂, and second CIGS target10B is formed from one or more of In₂Se₃ and Ga₂Se₃. As will beappreciated, three targets for CuSe₂, In₂Se₃ and Ga₂Se₃ could be used,or various combinations of precursors used in different targets toarrive at a desired value of x or selected ratio of chalcogens. Forexample, in a two target system as shown in FIG. 1, one target mayinclude at least one of CuSe₂, In₂Se₃ and Ga₂Se₃ with selenium and theother target may include at least the others of CuSe₂, In₂Se₃ andGa₂Se₃.

In other embodiments, a dopant such as Na, Li, Cr, Ni, or Ti is added tothe film by co-sputtering from a target consisting of the dopant or of acompound containing the dopant.

Sputtering from two or more CIGS targets 10A, 10B with differentstoichiometries allows the ratio of different constituents of the layerto be varied by adjusting the electrical power applied to the respectiveCIGS targets. Additionally, the composition of the film can be variedwith depth through the film's thickness by varying the power/controllingthe relative power applied to each target during deposition. This methodcan be applied, for example, to create a film with a tailored bandgap.

As will be appreciated, the exemplary method is not limited to two orthree targets. For example, as many as six sputtering targets may beused for simultaneous deposition.

The substrate 36 can be formed from any suitable material, such asglass, metal foil, plastic or the like. The substrate 36 may have one ormore intermediate layers deposited on it prior to layer 40. By way ofexample, FIG. 3 shows a semiconductor thin-film solar cell having amultilayer structure superposed on a substrate 36 in the followingorder, a back electrode 50, the exemplary thin film 40 as a lightabsorbing layer, an interfacial buffer layer 52, a window layer orlayers 54, 56 and an upper electrode 58. The substrate 36 may be, forexample, soda-lime glass, metal ribbon, or polyimide sheet and may beabout 1-100 mm thick. The back electrode 50 may be a molybdenum metalfilm and may be about 0.5-10 μm thick. The thin film 40 may be about 500nm-3 μm thick. The interfacial buffer layer 52 may be formed fromcadmium sulfide, and may be about 10-100 nm thick. The window layer 54,56 may be formed from zinc oxide and indium tin oxide and may be about0.1-1 μm thick. If the window layer 54, 56 is formed from zinc oxide,part or all of the films thickness may be doped with about 0.5-5 weightpercent aluminum. The upper electrode 58 may be a grid with a maximumthickness of about 0.5-10 μm.

To form such a structure, a substrate 36 is sputter coated withmolybdenum, forming the back electrode 50. Then the CIGS absorptionlayer 40 is deposited over the back by sputtering, as described above.Next, a chemical bath is applied to deposit cadmium sulfide 52, forminga heterojunction with the adsorption layer 40. Then, zinc oxide andindium tin oxide are sputter coated on to form a clear window 54, 56.The grid 58 is e-beam-evaporated on top. As will be appreciated, thestructure may include fewer, additional, or different layers from thosedescribed.

One advantage of the exemplary embodiment is that the exemplarydeposition method can eliminate the need for post-depositionselenization and/or annealing because the desired composition can becreated during co-sputtering. As a result of the elimination ofpost-deposition selenization and/or annealing, the proposed methodenables the deposition of the composition on substrates that cannottolerate high temperatures, such as plastics.

Another advantage is that the sputtering targets can be independentlycontrolled to create either multilayered or codeposited films. As anexample, a layer of only Se can be deposited, followed by a layer ofother materials. This process can be repeated several times.

The sputtering targets can be formed, for example, by hot pressing ordirect melting in a quartz ampoule or a crucible. For example, theexemplary targets 10, 10A, and 10B can be formed by the method describedin above-mentioned co-pending application Ser. No. 12/884,586, entitledSYNTHESIS OF HIGH-PURITY BULK COPPER INDIUM GALLIUM SELENIDE MATERIALS,by Vinh Q. Nguyen, et al., incorporated herein by reference. Inparticular, high purity elements for forming the CIGS source 10, such aselemental selenium, gallium, indium, and copper, are reacted together,optionally in the presence of one or more dopants. Some or all of theelemental materials used may be pretreated to reduce the concentrationof oxygen, e.g., by heating at a temperature above the melting point ina stream of hydrogen gas and/or by distillation. The materials arecombined in appropriate amounts in a reaction vessel which is evacuatedand sealed before heating the vessel in a furnace to a desired reactiontemperature, e.g., about 980-1100° C. Upon cooling of the reactionvessel, which can take place in the furnace or removed from it, thetarget 10 is formed as a crystalline monolith. The reaction vessel canhave an interior shape which is the same as that of the desired target.For example, the vessel can have an interior width (e.g., diameter), ofabout 5-15 cm. The vessel may be formed from a frangible, refractorymaterial, such as fused silica (quartz), which allows the cooled vesselto be broken open to release the already-formed target. The targets 10,10A, and 10B themselves, as well as CIGS films formed by the presentmethod using such a target or targets, can have a low oxygenconcentration, e.g., about 10 ppm, or less.

Other methods for forming the sputtering targets are contemplated. Forexample, the crystalline CIGS material formed in the reaction vessel isbroken up and compressed into a puck having the shape of the target.However, grinding of the CIGS material and forming it into a target canintroduce more oxygen, which is generally undesirable in the film.

A chalcogen target 12 can be formed in a similar way, e.g., by heatingthe chalcogen, optionally with dopants, in a target-shaped vessel. Thetarget shape is obtained on quenching of the melt. Optionally, asubsequent heat treatment is applied to control the degree ofcrystallinity and grain size.

Without intending to limit the scope of the exemplary embodiment, thefollowing examples describe co-sputtering of films.

EXAMPLES Example 1

High-purity bulk CIGS material is formed in a silica ampoule. The bulkmaterial is ground into a fine powder and hot pressed into a puck. Thepuck is machined into an approximately 7.6 cm diameter, 0.3 cm thicksputtering target. A commercial Se target is arc melted and cast, thenmachined to its final dimensions.

Deposition is carried out by RF magnetron sputtering in a sputter-upgeometry onto a substrate, as illustrated in FIG. 1. The sputteringprocess is carried out in an Ar atmosphere with a pressure of 5milliTorr and an Ar flow rate of 15 sccm. A first target 10 isCuln_(0.7)Ga_(0.3)Se₂. For this target, an energy density ofapproximately 2 W/cm² is used. A second target 12 is purepolycrystalline Se. For this target, an energy density of approximately0.7 W/cm² is used.

A molybdenum-coated soda lime glass substrate is used. The molybdenumlayer simulated the bottom contact in a photovoltaic device. Thesubstrate is positioned approximately 15 cm above the surfaces of thetargets 10, 12 and is rotated at a rate of approximately 15 rpm duringdeposition. The substrate is heated to a temperature of 400° C.Deposition is carried out for 12.5 hours. In other examples, thetemperature is modified from 100 to 550° C.

The resulting films are approximately 1.5 μm thick, but can be thickerdepending on choice of processing conditions and time.

FIGS. 4 and 5 show SEM cross sections of films made without (FIG. 4) andwith (FIG. 5) co-sputtering Se but with otherwise identical depositionparameters. As can be seen in the images, the films co-sputtered with Seexhibit distinct morphological differences compared to the filmsdeposited without it. In particular, the film is denser and has fewervoids.

Example 2

Films were formed as for Example 1 by a) keeping power to the targetsconstant at an energy density of approximately 2 W/cm² for the CIGStarget 10 and an energy density of approximately 0.7 W/cm² for the Setarget 12 throughout deposition and b) varying the power. For the CIGStarget 10 a constant energy density of approximately 2 W/cm² was used.For the Se target 12 the energy density was varied between 0 and 0.7W/cm². FIGS. 6 and 7 show the atomic concentrations through a crosssection of each film, determined by XPS. As can be seen from FIG. 6, arelatively homogeneous film is formed when the power ratio is constant.The atomic concentrations can be varied through the film (FIG. 7) whenthe power is varied.

As will be appreciated, the oxygen content of the film can be reducedover that shown in FIGS. 6 and 7 by using a bulk polycrystalline CIGStarget and a purified selenium target, as described in copendingapplication Ser. No. 12/884,586.

The exemplary embodiment has been described with reference to thepreferred embodiments. Obviously, modifications and alterations willoccur to others upon reading and understanding the preceding detaileddescription. It is intended that the exemplary embodiment be construedas including all such modifications and alterations insofar as they comewithin the scope of the appended claims or the equivalents thereof.

1. A sputtering apparatus comprising: a sputtering chamber; at least afirst target is positioned in the chamber, which is energizable tosputter a source of a CIGS material towards a substrate; and a secondtarget is positioned in the chamber, which is energizable to sputter achalcogen towards the substrate, whereby by energizing the first andsecond targets, a layer of CIGS material is provided on the substrate,the layer incorporating, within the CIGS material polycrystallinestructure, chalcogen from the second target.
 2. The sputtering apparatusof claim 1, further comprising a source of a sputtering gas connectedwith the chamber.
 3. The sputtering apparatus of claim 1, wherein thefirst and second targets are separately powered by a respective powersource.
 4. The sputtering apparatus of claim 1, wherein the first targetcomprises a bulk polycrystalline CIGS material.
 5. The sputteringapparatus of claim 1, wherein the first target and second target areangled to each other to sputter a common area of the substrate.
 6. Amethod for forming a solar cell comprising: sputtering first and secondtargets towards a substrate to provide a film comprising polycrystallineCIGS material on the substrate, the first target including a source ofCIGS material having an oxygen concentration of less than 10 ppm byweight and the second target including a chalcogen and having an oxygenconcentration of less than 10 ppm by weight, the chalcogen in the secondtarget being at a higher concentration than in the first target; andproviding first and second electrode layers on the substrate which arespaced by the film.
 7. A solar cell formed by the method of claim 6.